Acoustic imaging of objects in optically opaque fluids

ABSTRACT

The present invention is a method and an apparatus that can image objects immersed in optically opaque fluids using ultrasound in a confined space and in a harsh environment. If the fluid is not highly attenuating at frequencies above 1 MHz, where commercial ultrasound scanners are available, such scanners can be adapted for imaging in these fluids. In the case of highly attenuating fluids, such as drilling mud, then a low frequency collimated sound source is used.

PRIORITY STATEMENT

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Patent Application No. 61/255,014, filed on Oct. 26, 2009,the disclosure of which is incorporated herein by reference to thefullest extent consistent with the present disclosure.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.DE-AC52-06NA25396, awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND OF INVENTION

This invention relates to ultrasonic imaging. More particularly, thepresent invention relates to obtaining ultrasonic images of items thatare immersed in optically opaque and often, in addition, acousticallyattenuating fluid media.

There are situations where it is necessary to image objects that are inan environment, such as motor oil or crude oil, or mud or otheroptically opaque media. For example, there is a need to inspect fueltanks of ships or large boats that may hide contrabands. Such inspectionis typically carried out using a long stick to feel through the fuelcompartment to locate objects. Another example is to address the need inthe oil/gas industry to visualize the down-hole drilling environmentwhere it is necessary to monitor drill pipe, casing, and drillingcollars and also to identify and then retrieve dropped testing tools,logging tools, and tubing. Currently, there is not an efficient andreliable method of accomplishing this.

The current state-of-the-art is to lower a lead impression disc down thepipeline and to then try to get an imprint of the dropped object on thelead disc. The impression produced by this method is highly inadequateand does not provide much depth of field information. This method doesnot result in a detailed image of any kind. It is very difficult to tellwhat the dropped object is from the crude impression obtained by thismethod.

Another method recently introduced is to lower a regular optical cameradown the pipeline but this requires replacing the existing drillingfluid with clear water that takes several days in a regular well-boreand lots of adjustments before a useful image can be captured. Infraredimaging has also been explored and it has the ability to image a shortdistance through water and some low API crude oil but no imaging ispossible through heavy oil or drilling mud. The environment encounteredin well-bores for oil/gas exploration is rather harsh (corrosive fluids,high pressure and temperature) and the space available is confined,typically the size of the bore hole. It is also important to be able toobtain some depth of field information to understand the nature of theobject of just getting a surface image.

The present invention provides the much-needed ability to provide clearand identifiable images of objects in optically opaque and acousticallyattenuating fluids, such as drilling mud, heavy crude oil, brine/oilmixture, and bubbly fluids.

SUMMARY OF INVENTION

An aspect of the present invention is a method and an apparatus that canimage objects immersed in optically opaque fluids using ultrasound in aconfined space and in a harsh environment. If the fluid is not highlyattenuating at frequencies above 1 MHz, where commercial ultrasoundscanners are available, such scanners can be adapted for imaging inthese fluids. In the case of highly attenuating fluids, such as drillingmud, the requirements for the imaging system are a lot more stringent asthis requires a low frequency collimated sound source.

In one aspect of the invention the apparatus for imaging an object inopaque fluids hereof may include: at least one transducer for generatinga directed, ultrasonic sound beam having a chosen frequency and also forreceiving reflected or scattered sound from the target object. Thetransducer is mechanically scanned to direct the sound beam in atwo-dimensional pattern using a mechanical wobbler such that an image ofthe target object can be produced from the reflected signal throughconventional electronics.

In another aspect of this invention the apparatus for imaging an objectin opaque fluids hereof may include: an array of transducers that can beelectronically scanned in phased array manner to direct an ultrasonicbeam and electronically scan the beam over of an area. The same array isused for receiving the reflected and scattered signal from the object.This signal is processed to create an image of the object.

In a further aspect of this invention the apparatus for imaging objectsin opaque fluids in harsh environment hereof may include: at least onetransducer for generating a directed, ultrasonic sound wave having achosen frequency and also for receiving reflected or scattered soundfrom the target object. The transducer is protected inside a metal tubeso the imaging is performed through a metal plate.

In yet another aspect of the present invention the method for imaging ofan object within opaque fluids hereof may include the steps of:generating a directed and collimated ultrasonic sound wave having achosen frequency, wherein a low frequency ultrasonic sound wave iscreated by a parametric array by combining two high frequency soundwaves in an acoustic nonlinear fluid, such as Fluorinert 43, and thislow frequency wave is directed toward the object to be imaged; detectingthe reflected and scattered signal from the object via a symmetricallyplaced receiver transducer close to the source; and electronicallyanalyzing the signal to produce an image of the object.

Benefits and advantages of the present invention include providing aquick way image objects. Although the examples below use the presentinvention in the oil industry, the ability to image objects in opaquefluids may be applied to many industries, particularly the medicalarena.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of the instrument including the scanner headinside a protective metal tube.

FIG. 2 a is a picture of pacman shaped metal object and FIG. 2 b. is astill image from a real-time video obtained from the ultrasound scannerof a pacman shaped object immersed in motor oil.

FIG. 3 a is a picture of a metals spring and FIG. 3 b. Still image madefrom a real time video obtained from the ultrasound scanner of thespring in motor oil.

FIG. 4 is a schematic representation of the parametric source using FC43as the nonlinear medium and the ring-shaped receiver.

FIG. 5 is a schematic of the imaging system using the parametric source.

FIG. 6 shows the time-of-flight measurement used to produce the image ofthe object shown in the inset.

FIG. 7 a shows a picture of the object imaged using the parametricsource and the ring transducer. FIG. 7 b is an image of the object shownin FIG. 7 a reconstructed from the time-of-flight measurement in waterat 550 kHz.

FIG. 8 a is an image of the object shown in FIG. 7 a reconstructed fromthe time-of-flight measurement in water-based 10 ppg mud at 550 kHz.FIG. 8 b shows the image of the object shown in FIG. 7 a reconstructedfrom the time-of-flight measurement in water-based 100 ppg mud at 175kHz.

FIG. 9 shows a demonstration of depth of field of the imaging systemwhere a single line cross-section of multiple objects at various depthsis shown.

DETAILED DESCRIPTION

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing a particular embodimentof the invention and are not intended to limit the invention thereto.

The present invention includes an apparatus and method for acousticimaging in optically opaque fluids with a varied amount of acousticattenuation from moderately low (e.g., brine mixed with oil or lightcrude) to very high (e.g., drilling mud). Imaging in such media is verychallenging because of the conflicting requirements that need to besatisfied. For example, higher attenuation means shorter penetrationdepth and the attenuation usually varies as square of the frequency.Therefore, it requires lower frequency to obtain deeper penetration intothe fluid. However, lower frequency means higher beam spread thataffects the spatial resolution in the image. Lower frequency alsorequires larger size transducers and this is difficult to accommodate ifthe measurement needs to be made in a bore hole or in any other confinedspace. Moreover, for a higher quality image in terms of highersignal-to-noise ratio a higher frequency bandwidth is required.Unfortunately, a lower frequency transducer has a lower bandwidth. Basedon these competing requirements, an imaging system that is compact butprovides a low frequency narrow beam is required. In moderately lowattenuating fluid, sound can penetrate the fluid several tens ofcentimeters even at frequencies as high as 3 MHz. Commercially availableultrasound scanners operate in the frequency range above 1 MHz. However,the scanning head (the sensor part) is very fragile and cannot be usedin harsh environments, such as where there is very high pressure.Developing an imaging system capable of withstanding such environmentcan be very expensive. In one aspect of the present invention theexisting commercially available scanning head is encased in a sturdymetal pipe for protection but still performs its imaging functions evenwhen the sound beam from the scanning head has to go through a metalplate.

FIG. 1 schematically shows one embodiment of the present invention. Ascanner head is placed inside a metal protective casing 2. The image ofthe intended object 3 is shown on display 4. In one example, thescanning head of a commercial ultrasound scanner (the wobbler type)operating at 3.5 MHz was placed inside a steel pipe with a wallthickness of 2.5 mm and diameter 5 cm. However, any other type ofultrasound scanner may be used, including the phased array kind. In theexample described above, the space between the front of the scanner headand the faceplate of the pipe through which the sound beam exits wasfilled with coupling gel. One of ordinary skill in the art would likelyhypothesize that the arrangement of the present invention would severelydegrade the imaging capability of the scanning system for a variety ofreasons, such as multiple reflections within the plate thickness,refraction of the beam through the plate, and impedance mismatch betweenthe metal plate and the liquid in which the object is immersed. However,the results of experiments with the present invention indicate that auseful image can be obtained and the metal pipe provides protection tothe scanning head.

A photo of a metal object shaped as a Pacman that is 10 cm in diameteris shown in FIG. 2 a. In one experiment, the Pacman was immersed in10W-40 motor oil inside a 15 cm diameter pipe and was placed at thebottom of the pipe. One embodiment of the present invention was thenused to image the object. FIG. 2 b is a still image captured from areal-time video from the scanner output. As FIG. 2 b shows, the objectcan indeed be imaged using the present invention.

FIG. 3 a shows a 7.5 cm long metal spring as the target object to beimaged. The purpose of using the spring is to demonstrate the highspatial resolution possible with the method of the present invention.The spring is immersed in motor oil similar to the Pacman shown in FIGS.2 a and 2 b. FIG. 3 b is a still image captured from a real-time videoof the ultrasound scanner output and it shows that the spring can bequite clearly imaged even through the pipe wall 2 that the scanner head1 is encased in. The quality and resolution of the image is excellent.

Several experiments were conducted in which measurements were made inwater and also in drilling mud using the embodiment described in FIG. 1.Imaging in water was quite straightforward and the sound penetration inwater was several feet. However, imaging in drilling mud was verydifficult and the penetration depth was less than 2 cm. The object 3needed to be in very close proximity with the scanner head 1. Althoughthis approach works well for low attenuating fluid, it is not practicalfor imaging objects in drilling mud or other highly attenuating fluid.This is because the high frequency (3.5 MHz) of the scanner that doesnot penetrate beyond a few cm in the highly attenuating mud. It is notpossible to simply substitute the high frequency transducer used in thisscanner with a much lower frequency one as the beam spread will be verylarge, which will severely limit the spatial resolution. Typically, thefrequency needs to be lower than 600 kHz to be practical in terms ofproviding high quality image with sufficient depth of field (e.g., >15cm). Such low frequency transducers will be quite large (both diameterand thickness) and heavy and the simple mechanism of a typical wobblerwould not work. It is also not possible to devise a phased array imagingsystem that is small enough at such a low frequency because of the sizeof the transducer elements needed for the array.

This invention overcomes all these difficulties by using a parametricsound source that is highly compact but can provide a highly collimatedbeam at frequencies from as low as 50 kHz to 1 MHz. The design of oneembodiment of the sound parametric source is shown schematically in FIG.4. It consists of a piezoelectric disc 7. The disc 7 may beapproximately 5 mm in diameter. The disc 7 is air-backed and is enclosedin a hollow cylinder 6, which may be filled with a liquid (Fluorinert43) that has high acoustic nonlinearity (beta ˜7.5). The cylinder 6 maybe made of any solid. Other liquids within the Fluorinert family canalso be used. Fluorinert liquid (FC 43) has a very low sound speed (˜650m/s), which is a fraction that in water. If two high frequency (f₁ andf₂) sound waves 8 are allowed to propagate in this liquid in a collinearmanner, these two frequencies mix in this nonlinear medium and generateboth a difference frequency and a sum frequency. The sum frequency getsattenuated very quickly as the sound attenuation is proportional tosquare of the frequency. The two primary sound waves of high frequency(typically 1-10 MHz) are also rapidly attenuated in the liquid. Thisleaves a difference frequency sound beam to travel while maintaining thecollimation of the primary waves. For example, if a sound wave of 9 MHzis mixed with another wave at 8.5 MHz, the difference frequency soundwave will have a 500 kHz frequency but it will also have the beam spreadof the 8.5 MHz primary wave. This method is called parametric generationof low frequency collimated sound. The mixing length in the liquid forFluorinert family of liquids is limited by what is frequently called theshock length that is related to the primary frequency and the beta ofthe liquid. The higher the frequency shorter is the shock length. Theshock length represents a maximum length; beyond this length if too muchpower is applied to the transducer, the system will produce shock waveor higher harmonics. The shock length therefore limits the powergenerated in the difference frequency wave. Typically, the strength ofthe difference frequency signal is more than 1000 times less than thestrength of the primary signal. With a collimated beam this differencecan be much greater.

Another factor that limits the power of the difference frequency signalgenerated is the frequency step down factor. If the ratio of thedifference frequency to the primary frequency is more than 20 then thesignal strength of the difference frequency goes down rapidly as afunction of the step down ratio. Therefore, it is important to selectthe proper operating factors. If the difference frequency required isvery low, the primary frequencies need to be lower as well. For example,the shock length at 9 MHz in FC43 is approximately 1 cm and thus themixing length must also be approximately 1 cm. As a result, a compactsource is developed. For example, the length of the FC43-filledcylindrical tube used in one embodiment of this invention was 2 cm, andit is possible for it to be been half that length. However, the slightlylonger length used was to accommodate lower frequencies. This geometryallowed the generation of a difference frequency collimated signal withfrequency that could be varied from a low of 150 kHz to 3 MHz. This isan extremely high frequency bandwidth and cannot be obtained by anyconventional means.

A complete imaging system is described schematically in FIG. 5. This isfor illustration purposes only and may have many variations. This designshown in FIG. 5 follows closely the design of a traditional wobbler typeof imaging system where the sound source is wobbled with mechanicaldrive to allow it to scan an arc shaped path. However, this design canbe easily converted to a linear x-y scanning system as well. Themechanical scanning system allows the sensor head (the parametricsource) to move in two orthogonal directions covering a 2-dimensionalarea. The sound that is scattered or reflected from a target is detectedby a ring transducer that is placed around the mouth of the soundsource. This makes the system collinear. In another embodiment, smalltransducers may be placed around the source and connected together inparallel to act as the receiver. The receiver is selected for itsability to respond in the low frequency so that it can detect thedifference frequency signal. The two high frequency primary signals inthe form of tone bursts are generated by two separate DDS (directdigital synthesizers) modules at the same time and then combined beforebeing amplified by a power amplifier and then fed to the sourcetransducer which in the pictured embodiment is an air-backedpiezoelectric disc. The amplitude modulating a carrier signal can alsobe used instead of combining two separate signals. For better depthdetermination, the broadband characteristic of the source is utilized.

In one experiment, a fixed frequency tone-burst signal is combined witha frequency chirp signal. For example, a fixed frequency signal at 9 MHzand of 100 μs duration is combined with a chirp signal that may varybetween 8.4 MHz and 8.8 MHz and of 100 μs duration. This generates adifference frequency chirp signal between 200-600 kHz. The chirp methodprovides significantly higher spatial resolution for depthdetermination. In experiments, it was determined that a singlepiezoelectric disc works better than the conventional broadbandcommercial transducers, however either may be used. The received signalis also amplified before being sent to a DSP (digital signal processormodule). The DSP carries out all the data analysis and is controlled bya microcontroller. The processed data is displayed on a screen.

The data processing and image construction consists of determining thetime-of-flight signal at each physical point of scan on a 2-D surface.This is shown in FIG. 6. The inset shows a photo of the object beingimaged. The location of the return tone-burst from the various depths ofthe object provides the information for image reconstruction. Thebeginning of the tone burst is used to determine the depth information.However, for a more accurate image reconstruction, a cross-correlationis performed where the peak in the correlation identifies thetime-of-flight information. A frequency chirp approach allowssignificantly better time-of-flight determination through thecross-correlation approach as the signal-to-noise improvement isproportional to the product of signal duration and the frequencybandwidth. The DSP allows such cross-correlation calculations inreal-time.

FIG. 7 a shows a picture of the sample object being imaged. The objectis an aluminum block with machined sections of different depths toprovide a target for imaging. The length is 7 cm, the height is 6 cm,and the width is 4 cm. The reconstructed image from the time-of-flightdata is shown in FIG. 7 b. The measurement shown is made with the objectimmersed under water and at a difference frequency of 550 kHz.

The imaging under water-based mud of 10 ppg (pounds per gallon)consistency is presented in FIGS. 8 a & 8 b for difference frequencies550 kHz and 175 kHz, respectively. The image of the object is faithfullyreproduced even at such low frequencies. This is possible because of thecollimated beam produced by the parametric array method of sound beamgeneration.

Finally, the depth of field measurement capability of this imagingsystem is demonstrated by stacking multiple objects on top of each other(not shown) inside an 8 inch diameter filled with drilling mud of 10 ppgconsistency. In one experiment, the objects were at the bottom and thesource was placed 6.7 cm from the top surface. Therefore, the totaldistance between the source and the bottom surface on which the objectis placed is 23 cms. Only a single line scan along the object is shownfor clarity in FIG. 9. The y-axis of the plot is travel time and so thelongest travel time indicates the farthest point and in this case thebottom surface of the target. Measurements were made in oil-based mud aswell with similar success. The highest density mud used was 15 ppg forimaging.

This invention shows that it is indeed possible to image objectsimmersed in highly attenuating and optically fluids, such as drillingmud and good depth of field measurement is possible. There can be manyother applications of this invention as it can be used anywhere where itis important to have a low frequency collimated beam for greaterpenetration and imaging, such a biomedical applications, contrabanddetection, imaging under opaque fluids and harsh environment in generaletc.

The invention is not restricted to the illustrative examples describedabove. Examples are not intended as limitations on the scope of theinvention. Methods, apparatus, compositions, and the like describedherein are exemplary and not intended as limitations on the scope of theinvention. Changes therein and other uses will occur to those skilled inthe art.

1. An apparatus for imaging objects in opaque fluids comprising: atleast one transducer for generating a directed, ultrasonic sound beamhaving a chosen frequency and also for receiving reflected or scatteredsound from the target object; a scanning head; a protective casingplaced around the scanning head; coupling gel placed between the scannerhead and the protective casing; an acoustic imaging device; and anelectronic circuit to receive image data from the imaging device anddisplay it on a screen.
 2. The apparatus of claim 1, wherein theprotective casing is a metal pipe.
 3. The apparatus of claim 1, whereinthe scanner is the wobbler type.
 4. The apparatus of claim 1, whereinsaid electronic circuit comprises a computer, for example including aprocessor, memory, clock, display driver, and other components:
 5. Theapparatus of claim 1, wherein the imaged object includes tools.
 6. Theapparatus of claim 1, wherein the depth of field of the image isapproximately 1 foot.
 7. The apparatus of claim 1, wherein theresolution of the image is approximately 2 mm.
 8. The apparatus of claim1, wherein the at least one transducer is an array of transducers thatcan be electronically scanned in a phased array manner.
 9. An apparatusfor imaging objects in opaque fluids comprising; a collimated source; anacoustic imaging device; and an electronic circuit to receive image datafrom the imaging device and display it on a screen.
 10. The apparatus ofclaim 9 wherein the collimated source is broadband and has a frequencyrange of 50 kHz-1 MHZ.
 11. The apparatus of claim 9, wherein thecollimated source is made using Fluorinate 43 or any other liquid in thesame family.
 12. The apparatus of claim 9, further comprising: a ringtransducer around the collimated source.
 13. A method for imaging of anobject within opaque fluids comprising the steps of: generating adirected and collimated ultrasonic sound wave having a chosen frequency,wherein a low frequency ultrasonic sound wave is created by a parametricarray by combining two high frequency sound waves in an acousticnonlinear fluid; symmetrically placing a receiver transducer close to asource transducer; directing the low frequency wave is directed towardthe object to be imaged; and detecting the reflected and scatteredsignal from the object; and electronically analyzing the reflected andscattered signal to produce an image of the object.
 14. The method ofclaim 13, where the acoustic nonlinear fluid is Fluorinate
 43. 15. Themethod of claim 13, further comprising obtaining the travel time of thesignal from the target object back to the receiver transducer next tothe source transducer.
 16. The method of claim 15, wherein the image isconstructed from the travel time information.
 17. The method of claim13, further comprising the step of using a collinear measurement systemto provide an image of the object.
 18. The method of claim 13, whereinthe image may be produced by scanning the object in 2D arcs.
 19. Themethod of claim 13, wherein the image may be produced by linear x-yscanning.
 20. The method of claim 13, wherein the opaque fluid includesone of the following: bubbling liquids, motor oil, water and thick mud.